Lithographic printing plates are required to be able to be exposed and imaged efficiently with high resolution; they must be capable of printing uniformly onto various substrates to produce high quality images; and they must be capable of running hundreds of thousands of copies, all at low cost. Generally, printing plates are made from lithographic grade aluminum. The plates are treated to clean their surfaces; to grain or roughen the surface to improve the adhesion of a photopolymer coating; to anodize the grained surface to improve their durability; and to treat the surface to obtain a hydrophilic surface.
Typically, the initial step in the preparation of aluminum printing plates is to clean or etch the raw aluminum, usually with an alkaline etch, which can be followed with an acid etch. The aluminum plate is then grained to promote adhesion of subsequently applied layers, and to improve the ink-water balance on a printing press. Graining is suitably done using an electrochemical method, or a combination of mechanical and electrochemical methods, to provide a uniformly roughened surface having a topography on the order of up to one micron.
The grained aluminum is usually oxidized, or anodized, forming an aluminum oxide coating, such as by treating with phosphoric or sulfuric acids using an electrochemical process. The oxidized surface is then passivated, as with a silicate coating and/or a polyvinyl phosphoric acid coating, to inhibit aluminum corrosion. This step produces a smooth coating that is durable and has a low porosity.
An image forming layer, such as a photoresist, is applied over the treated aluminum plate. For laser thermal imaging, which can be computer controlled, the plate must be able to withstand deposition of the photoresist, imaging and development of the photoresist, without adhesion loss of the photoresist to the substrate. The printing plate is then baked to xe2x80x9csetxe2x80x9d the resist.
In addition, the photoresist must have a wide development latitude, that is, the development of the photoresist after exposure, must be as long as possible to provide as much of a difference as possible between exposed and unexposed areas of the resist. A wide development latitude results in the possibility of using an increased photospeed, so that the resist can be exposed rapidly, increasing its reliability and providing flexibility for the choice of materials and exposure to the end user.
U.S. Pat. No. 5,962,192, based on Ser. No. 08/666,169 and issued on Mar. 14, 2000, discloses particular positive photoresists. Although they are excellent for use on copper clad metal printing plates, they did not work as well with the above conventional aluminum printing plates. Complete removal of the photoresist was not achieved, and the development latitude or range was narrow.
Thus it is an object of the present invention to prepare aluminum printing plates that can be used with the above photoresists to prepare highly durable printing plates having developed photoresists that adhere well to the plate, that have a wide development latitude for long development time, that have a clear image and that can be used to make many hundreds of thousands of copies from a single printing plate.
The present method for preparing aluminum printing plates includes the following steps in sequence:
a) treating the plate to clean its surface;
b) chemically graining the aluminum to promote good adhesion of an overlying photoresist;
c) treating with nitric acid to remove smut on the surface;
d) spraying the plate with water at a temperature of 160-180xc2x0 F.;
e) treating the plate with an acetate-phosphate solution; and
f) treating the plate with a silicate solution prior to applying a photoresist layer.
The photoresists are used with computer controlled laser diodes to produce a printing plate having a wide development latitude.
After cleaning an aluminum printing plate, it must be treated to obtain a durable plate having a maximum development latitude, in addition to treatments that will provide good adhesion between the photoresist and the plate. The demarcation between exposed and non-exposed areas of the resist must be clear and sharp, the plate must be able to withstand numerous printing cycles, and the non-image areas of the plate must exhibit good ink repellency.
A commercially available lithographic grade aluminum printing plate about 0.008-0.020 inch thick, in accordance with the invention, is treated as follows.
a) the plate is cleaned to remove grease and dirt by immersing for about 30-45 seconds in an aqueous solution of sodium hydroxide (26-28 grams/liter) and about 3.7 grams/liter of sodium gluconate heated to a temperature of about 140xc2x0 F. The aluminum plate is then rinsed with de-ionized (hereinafter DI) water.
b) the cleaned aluminum is grained by immersing in a solution of ammonium bifluoride and ammonium nitrate for about 45 seconds, and heated to about 130xc2x0 F. The ammonium bifluoride is suitably employed at a concentration of about 50 to 125 grams/liter, preferably about 75 grams/liter, and the ammonium nitrate is used in a concentration of from about 1-10% of the ammonium bifluoride concentration, preferably about 10%. Nitric acid may be substituted for the ammonium nitrate. The aluminum plate is again rinsed in DI water.
c) once grained, the aluminum is sprayed with a 5-30% by volume solution of 42 Baumexc2x0 nitric acid to de-smut the surface.
d) the plates are then sprayed with hot water (160-180xc2x0 F.) to begin the formation of an oxide layer.
e) immediately after step d), the plates are immersed in a solution including about 10-12 grams/liter of sodium acetate, about 1 gram/liter of sodium carbonate and sufficient trisodium phosphate to bring the pH of the solution within a range of from about 10.5-11.5. After heating the solution to about 160-200xc2x0 F., the plates are immersed in the solution for about 30-60 seconds. This completes the formation of the oxide layer. The aluminum plates are again rinsed with DI water.
f) the oxidized aluminum plate is then silicated by immersing for about 30-45 seconds in a solution containing about 15-25 ml/liter of a silicate solution commercially available from PQ Corporation under their tradename STARTN. The solution was heated to about 150-180xc2x0 F. The plate was again rinsed with DI water.
Thus the present method omits a separate anodization step. However, the hot water rinse and acetate steps ensures that the resist will adhere to the metal printing plate, and permits the aluminum metal plate to behave in the same way as a prior art bi-metal plate of copper clad metal. Thus the anodization step and process baking steps can be omitted when using the present hot water rinse/acetate treatment. Because of the improved adhesion of the resist to the printing plate, the post-baking step can also be omitted. Thus the present method reduces the number of steps required to prepare a high quality printing plate having excellent adhesion, wide development latitude and great durability.
A positive photopolymer, such as described in U.S. Pat. No. 5,962,192, is then applied to the treated aluminum surface by spin coating. These positive photopolymers are made from an organoazide compound mixed with a suitable polymeric resin and one or more dyes that are sensitive to light emitted by the particular laser used for exposure. Additional ingredients, such as a pigment that improves the contrast between the resist layer and an underlying substrate, and surfactants, designed to adjust the texture of the resist so that it will form a smooth coating and will have a uniform thickness on the prepared aluminum plate, can also be added. Typically, lithographic resists of the present type have a dry thickness of about 40-80 microinches. The resist is mixed with a suitable organic solvent so that it can be applied to the printing plates as a thin film having a uniform thickness. The solids are generally present at a concentration of about 3-5% by weight in the organic solvent.
Polymeric resins suitable for use in the photoresist include polyvinyl formal resin and its derivatives, polymers and copolymers of acrylates and methacrylates, styrene and the like.
Suitable organoazide compounds can be mono- or multi-functional and generally have more than one azide group. The azides are used together with suitable dyes, such as infrared absorbing dyes, that are photosensitive to the light emitted by the particular laser used for patterning the resist. It is believed that high intensity laser light excites electrons in the dye absorbers so that the laser light is transformed into heat energy. The heat energy is transmitted to the organoazide compound, dissociating the azide, and making it soluble in a developer solvent. The dye chosen for the present resists must be sensitive to the frequency of the laser used to expose the resist, and must be able to absorb the radiation from the laser and convert it to heat. Various dyes are available commercially that are sensitive to infrared light having various emission frequency ranges for use with laser diodes, infrared lasers, YAG lasers, carbon dioxide lasers and the like. What is important is that the dye be sensitive in the same region of the spectrum as the light emitted by the light source used to pattern the resist.
Flood exposure by ultraviolet light can be used to improve the insolubility of the insoluble portions of the resist after exposure to the patterning light. The organoazides remaining on the substrate will cross link in the presence of ultraviolet light, making them less permeable to the developer solvent. This can suitably be done prior to laser exposure.
The resist is then developed using conventional developer solutions and equipment. The developer solubilizes the exposed regions of the resist, and rinses it away. The presence of a pigment is advantageous because one can determine visually when the resist has been removed down to the substrate in the image exposed regions.
The resultant aluminum printing plates will carry printing inks, which can be transferred to another medium, such as paper.
The invention will be further described in the following example, but the invention is not meant to be limited to the details described therein.
In the Example, adhesion of the resist was determined by soaking the plate in FS-70 fountain solution (Printing Developments, Inc) for three days. A clear adhesive tape was applied to the plate and the tape peeled away. Adhesion is deemed to be xe2x80x9cgoodxe2x80x9d if no resist is removed with the tape, specifically in the fine screen areas.
Development latitude was determined by the range of development time required to produce a clean metal surface in the non-image areas, without degradation of the resist in the image areas. The printing plates were then used to reproduce the pattern from the printing plate onto another surface, such as paper.